1. Field of the Invention
This invention relates to a control circuit for a light-emitting device.
2. Description of the Related Art
In recent years, a light-emitting diode (LED) has come into widespread use as a light-emitting device for lighting to replace an incandescent electric lamp from a standpoint of energy saving and the like.
FIG. 7 is a circuit diagram of a conventional control circuit 200 for a light emitting device directed toward improving a power factor. The control circuit 200 includes a rectification circuit 50, a reference voltage generation circuit 51, a comparator 52, an RS flip-flop 53, a choke coil 54, a regeneration diode 55, a switching device 56 and a resistor R0 for current detection.
When an alternating current (AC) input voltage Vin is applied to input terminals of the rectification circuit 50, the input voltage Vin is full-wave rectified by the rectification circuit 50. A full-wave rectified voltage Vrc is supplied to an anode of an LED 60 as a drive voltage. A cathode of the LED 60 is connected to a ground through the choke coil 54, the switching device 56 and the resistor R0. A terminal voltage of the resistor R0 is applied to a non-inverting input terminal (+) of the comparator 52 as a comparison voltage Vcmp.
The reference voltage generation circuit 51 is composed of resistors R1 and R2 connected in series between an output terminal of the rectification circuit 50 and the ground, and generates a reference voltage Vref by dividing the full-wave rectified voltage Vrc. The reference voltage Vref is applied to an inverting input terminal (−) of the comparator 52. Waveforms of the AC input voltage Vin, the rectified voltage Vrc and the reference voltage Vref are shown in FIG. 8.
The comparator 52 compares the comparison voltage Vcmp with the reference voltage Vref. A comparison output voltage Vcout from the comparator 52 is at an H level when the comparison voltage Vcmp is larger than the reference voltage Vref, and at an L level when the comparison voltage Vcmp is smaller than the reference voltage Vref. The comparison output voltage Vcout is applied to a reset terminal R of the RS flip-flop 53.
A trigger pulse Vtr is periodically inputted to a set terminal S of the RS flip-flop 53. The RS flip-flop 53 outputs a flip-flop output voltage Vfout from its output terminal Q. The flip-flop output voltage Vfout is applied to a gate of an N-channel type MOS transistor that makes the switching device 56.
The RS flip-flop 53 is set in response to the trigger pulse Vtr, and is reset in response to the comparison output voltage Vcout from the comparator 52, as shown in FIG. 9.
When the RS flip-flop 53 is set in response to the trigger pulse Vtr, the flip-flop output voltage Vfout is turned to the H level and the switching device 56 is turned on. As a result, the LED 60 is provided with a current flowing through the choke coil 53, the switching device 56 and the resistor R0, and the LED 60 is turned on. The current flows through the resistor R0 at that time, and the comparison voltage Vcmp that is the terminal voltage of the resistor R0 is raised as a result. When the comparison voltage Vcmp becomes larger than the reference voltage Vref, the comparison output voltage Vcout is turned to the H level to reset the RS flip-flop 53. At that time, since a change in the current flowing through the choke coil 54 is proportional to an electric potential difference between both ends of the choke coil 54, there is required a certain period of time after the switching device 56 is turned on and before the comparison voltage Vcmp becomes larger than the reference voltage Vref.
When the RS flip-flop 53 is reset, the flip-flop output voltage Vfout is turned to the L level and the switching device 56 is turned off. As a result, the current provided to the LED 60 through the switching device 56 is cutoff. When the switching device 56 is turned off, the comparison voltage Vcmp is lowered because no current flows through the resistor R0. Then, the comparison output voltage Vcout from the comparator 52 returns to the L level when the comparison voltage Vcmp becomes smaller than the reference voltage Vref.
The control circuit 200 can control average intensity of light emission of the LED 60 by controlling the current flowing through the LED 60 as described above. A regeneration diode 55 is connected in parallel with the LED 60 and the choke coil 54 so that energy stored in the choke coil 54 is returned to the LED 60 when the switching device 56 is turned off.
This kind of control circuit for the light-emitting device is disclosed in Japanese Patent Application Publication No. 2010-245421.
A voltage of AC power supply for households differs from area to area or country to country, and varies in a range between 100V and 200V, for example. As a result, there is a problem with the conventional control circuit 200 that when amplitude of the AC input voltage Vin increases from 100V to 200V, for example, amplitude of the reference voltage Vref increases accordingly to increase the current provided to the LED 60, as shown in FIG. 10.
That is, when the amplitude of the AC input voltage Vin is increased, the amplitude (peak voltage) of the reference voltage Vref is also increased accordingly, since the reference voltage Vref is a divided voltage of the rectified voltage Vrc that is generated by full-wave rectifying the AC input voltage Vin.
As a result, the period of time after the switching device 56 is turned on and before the comparison voltage Vcmp becomes larger than the reference voltage Vref is increased. Therefore, a period of time after the RS flip-flop 53 is set by the trigger pulse Vtr and before the RS flip-flop 53 is reset by the comparison output voltage Vcout from the comparator 52 is also increased and a period of time during which the LED 60 is provided with the current flowing through the switching device 56 is increased accordingly. (Refer to the flip-flop output voltage Vfout and the comparison output voltage Vcout indicated by dashed lines in FIG. 9.)